CHIME:
Chime is a strange but fun hybrid of Tetris and Lumines, set to music. Very addicting and fun, but you can't add your own tunes, which is missed potential there.
Available on: XBLA, PSN, PC (Steam)
BEAT HAZARD:
A top-down shmup in the vein of Geometry Wars with a dash of bullet-hells on LSD, Beat Hazard is not only a really fun and challenging (on the harder difficulties) game, but it is also a developmental miracle. Created by Cold Beam Games, which consists of one person (That's right, ONE PERSON), Beat Hazard is amazing in terms of not only fun factor, but content as well. There's online multiplayer, multiple gametypes, many different types of enemies, each with their own strategy, boss fights, and a fully-featured perks and leveling system. You can choose any mp3/aac/flac music file, and the on-screen action (amount of enemies, speed of projectiles, even the power of your own weapons) is linked to the frequencies of the song. It's simply brilliant, and amazing in the bang-for-your-buck sense.
Available on: XBIG (the original, highly outdated and as such, barebones release), PSN, PC, Mac OSX, Steam
More to come